Highly water absorbing agents have been developed in recent years and used widely in disposable diapers, sanitary napkins, and like absorbing articles and also as water retaining agents for agriculture/horticulture and construction purposes, principally for disposable products. Raw materials, including various monomers and hydrophilic polymers, are being suggested for the manufactured of the water absorbing agent. Especially, polyacrylic acid- or polyacrylate-based water absorbing resins produced from an acrylic acid and/or a salt thereof as a monomer are the most popular for industrial uses for their excellent water absorbing capability.
Conventionally known, desirable water absorbing properties for the water absorbing agent include: centrifuge retention capacity, absorption capacity against pressure, water absorption rate, liquid permeability under no load, liquid permeability under load, shock resistance, urine resistance, fluidity, gel strength, particle size, and many other properties (parameters). In addition, various definitions (parameter measuring methods) are being suggested for the same physical property (e.g., centrifuge retention capacity) from various viewpoints.
Since the water absorbing agent is used principally for diapers, sanitary napkins, and other sanitary articles, if the water absorbing agent in powder form is actually used with white pulp in a sanitary article, the water absorbing resin is required to be white upon shipment from a factory in order to avoid giving consumers an uncomfortable feeling due to coloring. A water absorbing agent is typically white powder. It is known however that the agent will color over time (change from yellow to brown) after shipment, during storage and transport and also when used in sanitary articles. Hence, the absorbing articles are required to remain white after being stored for an extended period of time. The coloring problems are increasingly challenging due to the recent trend of rising usage (wt %) of water absorbing resin in sanitary articles.
There is a large literature addressing the coloring problems of the water absorbing agent. Patent document 1 suggests a method of polymerizing an acrylic acid and/or a salt thereof using hydroxy peroxide and a reducing agent and thereafter processing the product with a silane coupling agent. Patent document 2 suggests a method of processing a water absorbing resin with an organic phosphorous compound or a salt thereof. Patent document 3 suggests a method of controlling the total amount of hydroquinone and benzoquinone in an acrylic acid to or below 0.2 ppm. Patent document 4 suggests a method of adding an inorganic reducing agent to a water absorbing resin. Patent documents 5, 6, 7 suggest a method of adding an organic carboxylic acid or a salt thereof to a water absorbing resin. Patent document 8 suggests a manufacturing method of polymerizing an acrylic acid containing tocopherol as a polymerization inhibitor. Patent documents 9, 10, 11 suggest a manufacturing method of adding a metal chelating agent in the manufacture of a water absorbing resin. Patent document 12 suggests a manufacturing method of adding acid in the manufacture of a water absorbing resin.
All these methods degrade physical properties and lead to additional cost, without providing sufficient improvements in the coloring problems. Also, some compounds used could be hazardous.    Patent document 1: Japanese Unexamined Patent Publication No. 4-331205/1992 (Tokukaihei 4-331205)    Patent document 2: Japanese Unexamined Patent Publication No. 5-86251/1993 (Tokukaihei 5-86251)    Patent document 3: U.S. Pat. No. 6,444,744    Patent document 4: International Patent Application Published under the PCT, No. WO2000/55245    Patent document 5: Japanese Unexamined Patent Publication (Tokukai) No. 2000-327926    Patent document 6: Japanese Unexamined Patent Publication (Tokukai) No. 2003-52742    Patent document 7: Japanese Unexamined Patent Publication (Tokukai) No. 2005-186016    Patent document 8: International Patent Application Published under the PCT, No. WO2003/53482    Patent document 9: Japanese Unexamined Patent Publication (Tokukai) No. 2003-206305    Patent document 10: Japanese Unexamined Patent Publication (Tokukai) No. 2003-206381    Patent document 11: U.S. Pat. No. 5,610,208    Patent document 12: International Patent Application Published under the PCT, No. WO2005/92956